The present invention relates to a dry etching method which utilizes a plasma produced by electron cyclotron resonance and to a dry etching apparatus which is used in the method.
The attainment of higher density in a semiconductor integrated circuit is closely related to the line width of its transistors, wires, and the like. By reducing the line width of these components, it becomes possible to produce a semiconductor integrated circuit with a micropattern having the line width of 1 .mu.m or less. The realization of such a micropattern is largely attributed to the development of two types of techniques: photolithography and dry etching.
Plasma-assisted dry etching methods utilize the phenomenon that when a material to be etched is placed in a reactive plasma or a radical, each produced by applying a radio-frequency (RF) electric field to an appropriate gas, the material is subjected to etching. To form a micropattern, a photoresist pattern is typically used as a mask material.
As an embodiment of the dry etching methods mentioned above, an ECR dry etching apparatus which utilizes a plasma produced by electron cyclotron resonance (ECR) has recently been disclosed and manufactured on an industrial basis.
FIG. 6 is a view diagrammatically showing the ECR dry etching apparatus which utilizes electron cyclotron resonance generated by the interaction between a microwave and a magnetic field. In FIG. 6, 1 denotes a plasma generation chamber in which the pressure is regulated at an appropriate value (several mTorr). A reactive gas is supplied to the plasma generation chamber 1 through a supplying line 2 and exhausted therefrom through an exhaust line 3. To the plasma generation chamber 1 is connected a microwave generator 5 via a waveguide 4 with a rectangular section. The microwave energy generated by the microwave generator 5 is supplied to the reactive gas which has been introduced to the plasma generation chamber 1.
Around the plasma generation chamber 1 is provided magnetic coils 6, which generate a magnetic field in the plasma generation chamber. Electrons in the plasma generation chamber 1 are accelerated by electron cyclotron resonance which results from the interaction between the microwave generated by the microwave generator 5 and the magnetic field applied to the plasma generation chamber 1 by the magnetic coils 6. The accelerated electrons then produce a plasma.
Underneath the plasma generation chamber 1 is provided a reaction chamber 7 which is connected to the plasma generation chamber 1. The plasma generated in the plasma generation chamber 1 is introduced to the reaction chamber 7. In the lower part of the reaction chamber 7 is provided a sample stage 8 on which a semiconductor substrate 9 with a resist pattern is placed as a material to be etched.
The operation of the ECR dry etching apparatus of the above-mentioned structure will be described below.
When the microwave energy at the frequency of 2.45 GHz is supplied to the plasma generation chamber 1 by the microwave generator 5, the reactive gas in the plasma generation chamber 1 is ionized to produce electrons and ions. An magnetic field of 875 G is also supplied to the plasma generation chamber 1 by the magnetic coils 6 provided around the plasma generation chamber 1. The interaction between the microwave and the magnetic field causes electron cyclotron resonance in the plasma generation chamber 1, which allows the electrons therein to absorb the electromagnetic energy of the microwave and to be accelerated. The electrons thus accelerated exhibit a circular motion at high speeds in the plasma generation chamber 1.
The acceleration of the electrons is carried out most efficiently by electron cyclotron resonance when the strength of the magnetic field applied to the plasma generation chamber 1 by the magnetic coils 6 is 875 G and the frequency of the microwave is 2.45 GHz in the plasma generation chamber 1. The electrons in a high-speed circular motion collide with the reactive gas, which generates a plasma with a high density in the plasma generation chamber 1. By diffusion and by an electric field, the resulting plasma is transferred from the plasma generation chamber 1 to the reaction chamber 7 along the magnetic lines formed by the magnetic coils 6, so as to be used in the thin-film formation or the etching treatment on the surface of the semiconductor substrate 9 placed in the reaction chamber 7.
In the above-mentioned ECR dry etching apparatus utilizing electron cyclotron resonance, the plasma is produced by causing electron cyclotron resonance in the plasma generation chamber through the interaction between the microwave energy and the magnetic field, causing a high-speed circular motion of the electrons due to the electron cyclotron resonance in the plasma generation chamber, and then causing collisions of the electrons in a high-speed circular motion with the reactive gas.
The microwave energy is supplied to the electrons most efficiently when the frequency f of the microwave and the strength B of the applied magnetic field fulfill the following condition of electronic cyclotron resonance (hereinafter referred to as the ECR condition): EQU f=q.times.B/2.pi.m (1)
wherein q (=1.602.times.10.sup.-19 C) denotes the unit charge and m (=9.110.times.10.sup.-31 kg) denotes the mass of the electrons. The optimum value of the microwave frequency f is 2.45 GHz when the strength B of the magnetic field produced by the magnetic coils is 875 G.
However, it is practically difficult to obtain a uniform strength of magnetic field which is applied by the magnetic coils to the plasma generation chamber. The strength of the magnetic field is relatively strong in the central portion of the plasma generation chamber while it is relatively weak in the marginal portion thereof. Consequently, the plasma density is relatively high in the central portion of the plasma generation chamber because the electrons in the central portion absorb the microwave energy efficiently, while it is relatively low in the marginal portion of the plasma generation chamber because the electrons in the marginal portion cannot absorb the microwave energy efficiently.
As described above, the distribution of the plasma density in the plasma generation chamber is not uniform in the conventional ECR dry etching apparatus, so that the amount of reactive ions which reach the surface of the semiconductor substrate placed in the reaction chamber is not uniform, either, which makes it difficult to treat the surface of the semiconductor substrate uniformly.
To overcome the difficulty, Japanese Laid-open Patent Publication No. 63-251026 discloses a plasma treatment apparatus in which the waveform of an electric current supplied to the magnetic coils is varied with the passage of time to form a rotary magnetic field which rotates the plasma current in the plasma generation chamber, thereby intending to realize a uniform plasma density.
However, it is extremely difficult in the abovementioned plasma treatment apparatus to control the distribution of the magnetic field so that the whole plasma can efficiently absorb the electromagnetic energy of the microwave, because the magnetic field is formed in the plasma in the plasma generation chamber. Hence, the realization of the uniform plasma density in the above-mentioned plasma treatment apparatus has its own limitations.
Moreover, though the plasma density is relatively high in the central portion of the plasma generation chamber because of the stronger magnetic field there, it is relatively low in the marginal portion of the plasma generation chamber because of the weaker magnetic field there, so that the overall plasma density throughout the plasma generation chamber is not increased.